Trees have an integral part in the green street stormwater strategy. Essentially, they help to collect rainwater in the canopy and then direct that water to the ground – allowing the water to infiltrate and recharge the groundwater table in a controlled manner. Water stored in the canopy is transpired back to the atmosphere.

The key factor impacting the tree’s ability to peform these tasks effectively is inadequate soil volume. Trees often have their root movement restricted within tree box installations. Highly compacted soils from pavement installation also impedes the roots from growing to full potential. The compacted soil acts as a deterent to stormwater infiltration, as well. The net effect is that the trees are smaller with reduced canopies.

Research by the Urban Forestry Department at Virginia Tech focused on the use of structural soils in the pavement surrounding the trees. The structural soil was shown to provide necessary strength for support of the pavement. More importantly, the porosity of the soil allowed for more infiltration, water storage, and provided sufficient soil volume to allow the trees to grow.

A .pdf of the presentation on how trees and structural soils work together to effectively manage stormwater can be found here.

When designed and constructed properly, vegetated swales can be one of the most effective BMPs available in the stormwater engineer’s toolbox. Due to its linear structure, it is quite effective at treating road runoff in residential areas. However, if it is to be incorporated into an urban street setting with large impervious areas, an important factor to keep in mind is that it should either be constructed in series or be used as a pretreatment feature ahead of other BMPs. Some of the benefits of vegetated swales include:

ability to improve water quality by reducing sediments and pollutant loads

ability to slow runoff velocities and promote infiltration

more cost-effective and easier to maintain than curb and gutter system

The key is proper design. Attention must be given to soil profile – in particular, depth to groundwater, depth to bedrock, and permeability of underlying soils. Following a storm event, a swale should be fully drained within 24 hours, so promoting infiltration is critical.

Another important factor in design is the selection of proper vegetation. Native plants and grasses should be selected to ensure long-term success of the swale. The vegetation acts as an impediment to flow in the swale. By slowing down the runoff, it allows for treatment and infiltration within the swale. It is vital that the vegetation take hold in the soil to ensure that the sloped sides will not erode.

Maintenance of a properly-designed vegetated swale is relatively simple. Once vegetation has matured and taken hold, grasses should be kept at 3 to 4 inches in height. Following storm events, swale bottoms should be inspected and cleaned of debris and sediment buildup to ensure best performance. Special attention should be given to any areas within the slope that may be underperforming and leading to erosion.

A good resource to review before considering incorporating vegetated swales in your green street design can be found through the EPA stormwater BMP fact sheets.

The first approach in green street design comes before consideration of any BMPs. In the planning stages, a look at how to reduce the overall impervious area is going to make the biggest impact on effective stormwater management. According to a 1995 Center for Watershed Protection study, nearly half of all impervious cover in residential developments comes from streets. Narrowing of streets can offer as much as a 20% reduction of this impervious cover.

Addressing the misconceptions

Considerations such as emergency and large vehicle access, parking, and safety are often assumed to be at loggerheads with narrow street design. When designed properly, fire and emergency services vehicles have the ability to manuever successfully. This has also been the case with larger service vehicles like garbage and delivery trucks. Parking demands in most residential areas typically can be satisfied through use of one parking lane (rather than two) coupled with driveways. Safety studies have indicated that narrower streets have lower auto and pedestrian accident rates primarily due to speed reduction.

Obvious benefits

The significant reduction in impervious cover is going to help control maintenance costs. An even bigger savings is going to be realized when installing narrower streets. By reducing the overall width of a street by even one foot, $10,000 is saved per residential mile. Stormwater management savings associated with this street reduction should be significant, as well.

Zoning roadblocks

The challenge to implementing narrow street design comes when facing zoning laws which force the hand of many designers to follow street width minimums. While some municipalities have adopted narrower street width requirements, greater steps will need to be taken to involve more decision-makers in the process. Going forward, studies of successful narrow street programs in cities such as Portland, Oregon, need to be published to help build the case for these decision-makers.

A strong resource to include in your library is the Green Streets website created and maintained by the Low Impact Development Center. With respect to green infrastructure, the LID center has been leading the way in research, design, guidance documents, training, and sustainable stormwater planning. The focus of the Green Streets website is to outline the five major approaches in design. Over the next several weeks, I would like to touch on each of these approaches in more detail.

Examples and photos of the five green street design approaches are found at the website and highlighted below:

Alternative street design – One of the best ways to address the stormwater problems begins before the process of considering BMPs. Through careful planning and consideration of existing hydrologic features, overall impervious area can be reduced.

Vegetated swales – These are successful at reducing flow through overall channel roughness, reducing volume through infiltration, and improving water quality through vegetative and soil filtration.

Bioretention features – A good feature to include in street design is a tree box, which forms a bioretention cell that reduces volume, attenuates flow, and treats the stormwater through biological/chemical reactions in the mulch, soil, and root zone.

Permeable pavement – Success of permeable pavement systems is well-documented. While providing structural support, these systems allow runoff storage, as well as pollutant removal through filtration.

Sidewalk trees – Considering their ability to reduce stormwater runoff, attention needs to be paid to the overall soil volume provided to trees. Inadequate tree box size and overcompaction of the soil during pavement installation are cited as two examples threatening the ability of trees to properly assist in reducing runoff.